Telecommunication techniques of today, for example as within the ATM-technique, impose higher and higher requirements on data transmission rates. In particular this concerns switches since they are to perform switching, multiplexing, etc., which can require even higher internal transmission rates. It can become interesting to more and more use rates in the range above Gbit/s and port for a switch within a near future, that is different from the presently highest used rates of 155 and 622 Mbit/s.
ATM-switches of various kinds are known, but they have usually a too complex structure to be successfully used for high transmission rates. A few examples of proposed designs are presented hereinafter.
The published International patent application WO-A1 95/01031 discloses a switch for receiving and forwarding data cells. The switch contains a multitude of ports connected to user terminals. A switch core 4 connects the ports and allows communication between them. The switch ports contain devices for providing the cells with information in regard of the selected path through the switch. An information part 14, "tag", see FIG. 2, associated with the cell, is created in the input port, at which the cell arrives. This part contains information on the selected route through the switch core and has a bitfield for each column of the core. Such information, associated with a considered cell, can be transmitted in advance to the switch core. A control unit ("scheduler") in the switch core receives the routing information and decides at which times different cells are to be transmitted from the input ports to the output ports in order not to interfere with transmission of other cells. Buffer memories for intermediately storing cells have only to be arranged in the output ports and not in the very core, resulting in a simple structure thereof. The information on the path for a considered cell is thus transmitted together with another cell that is received some cell cycles before the considered cell while it is intermediately stored in the input port.
In U.S. Pat. No. 4,623,996 a packet-switching node is described that communicates data packets containing information ("routing tag signals") in regard of the output to which the packets are to be forwarded. A multitude of queue selectors are individually connected between a multitude of input ports and output ports. Moreover, the switching node contains storage units ("queue sets") connected to each input port. Each queue selector sorts the data packets that are transmitted to the selector node, according to the information which determines each output port. With the storage units different control units ("arbitrators") are connected that guides the packets to the indicated output port. A queue selector is arranged for each input and further, it is connected to several storage units ("queue sets") specific to the input, one unit for each output.
In the published European patent application EP-A1 0 405 042 a cell-based switch of multi-stage type is disclosed, the output port being determined by a control word that is created in a control unit 36. The control word contains the address of the output port for the data cell to be transmitted and routing information that is divided for the respective switch stage. In FIG. 8 it is illustrated how the control word is processed. A unit 83 for selection of output port is used. Outgoing data cells are buffered individually in a "transmit adapter" for each output port.